The invention relates to an apparatus for carrying out DNP-NMR measurements on a sample, with a magnet configuration for producing a magnetic field in a first working volume, wherein the magnet configuration produces a stray field in a second working volume in the direction of an axis with at least one magnetic field gradient of first or higher order in the direction of the axis, wherein the axis extends through the second working volume, with a device for measurement of magnetic resonance (MR) signals of the sample from the first working volume, with a DNP excitation device for DNP excitation of the sample in the second working volume, and with a positioning mechanism for transferring the sample between the second working volume and the first working volume.
Such a configuration is known from WO 2005/114244 A1.
Nuclear magnetic resonance (NMR) spectroscopy is a commercially widespread method for characterizing the chemical composition of substances. However, this method has the disadvantage that it is not very sensitive because the polarization of the atomic nuclei is only weak even in large magnetic fields. To improve the signal-to-noise ratio, either the noise can be reduced or the signal increased. Over the past few years, great efforts have been made to suppress the noise. Cooled probeheads have been developed for this purpose.
To increase the signal, stronger magnets can, for example, be built. Alternatively, the so-called DNP (dynamic nuclear polarization) method can be deployed. In this case, the electrons of the sample that are strongly polarized in the background field of the magnet are excited into electron paramagnetic resonance (EPR) by radiation in the microwave range. Because of interaction of the electron spins with the atomic nuclei, their spins are also strongly polarized. While the polarization of the atomic nuclei in equilibrium at room temperature in a background field of 10 Tesla is typically a few 10 ppm, it can be as much as several percent with DNP, the sample having to be frozen and reheated for this purpose. It is crucial to conduct the NMR experiment before this polarization is lost.
Basically, there are two ways of deploying DNP: Either the EPR pre-polarization is performed in the NMR working volume or the sample is pre-polarized in a second working volume and then transported into a first working volume for conducting the NMR experiment. The former method has the disadvantage that, because of the typically very large field strength in the NMR working volume, a very high-frequency radiation is required for the EPR excitation, which is technically very difficult. In the latter case, to which this invention relates, there must be a mechanism that transports the sample from the second working volume into the first working volume. This transporting mechanism must operate as rapidly as possible so that the sample is still sufficiently polarized for the NMR experiment. It is therefore advantageous if the two working volumes are as close together as possible. In the stray field of an NMR magnet configuration, there is usually an area where the field strength is suitable for DNP excitation.
For the sample to be adequately pre-polarized, the magnetic field in the second volume must be sufficiently homogeneous. The stray field of an NMR magnet does not meet this requirement and must therefore be corrected. One solution to this problem is disclosed in WO 2005/114244 A1. There it is proposed that a superconducting magnet be located around the first working volume and a further superconducting magnet be located around the second working volume. The further magnet has the task of homogenizing the field in the second working volume but must not impair the homogeneity in the first working volume.
One essential disadvantage of the configuration according to WO 2005/114244 A1 is that a conventional NMR magnet system cannot be upgraded in this manner, meaning that the entire DNP-NMR magnet system has to be developed anew. If it is a superconducting magnet configuration, the cryostat normally also has to be changed.
Moreover, the mutual influence of the two magnets is also problematic. In particular, the magnetic force between the magnets can become so large that the further magnet around the second working volume has to be shielded. This is possible both actively by the use of superconducting coils and passively with the aid of ferromagnetic material, as described in WO 2005/114244 A1. On the other hand, the field homogeneity in each working volume must not be influenced too strongly by the other magnet. All these conditions can lead to complicated coil configurations.
The object of this invention is, by contrast, to propose an apparatus with a second working volume for DNP-NMR excitation, in which the homogenization of the magnetic field in the second working volume is achieved by simple technical means, so that the apparatus can also be obtained by upgrading a conventional NMR magnet configuration.